1. Field of the Invention
The present invention relates to an active matrix type organic light emitting diode (AMOLED) device, and more particularly, to a thin film transistor for use in a dual panel type organic light emitting diode device.
2. Discussion of the Related Art
Among flat panel displays (FPDs), organic light emitting diode (OLED) devices have been of particular interest in research and development because OLED devices are light-emitting type displays that have a wide viewing angle as well as a desirable contrast ratio, as compared with liquid crystal display (LCD) devices. Since a backlight does not need to be provided in conjunction with such OLED devices, the size and weight of OLED devices are small, as compared to other types of display devices. OELD devices have other desirable characteristics, such as low power consumption, superior brightness and fast response time. When driving OLED devices, only a low direct current (DC) voltage is required while obtaining a rapid response speed. Because OLED devices are entirely formed of materials in a solid phase arrangement, unlike LCD devices, OLED device are sufficiently strong to withstand external impacts and also have a greater operational temperature range. Moreover, fabrication of an OLED device is a relatively simple process with a few processing steps. Only deposition and encapsulation apparatuses are necessary for manufacturing the OLED devices. Accordingly, it is much cheaper to produce OLED devices compared to LCD devices or plasma display panels (PDPs)
In an active matrix organic light emitting diode (AMOLED) device, a voltage applied to the pixel and a charge for maintaining the voltage is stored in a storage capacitor from the applied voltage. This allows for a constant voltage driving the AMOLED device until a voltage for a next frame is applied, regardless of the number of the scanning lines. As a result, since an equivalent brightness is obtained with a low applied current, an AMOLED device having low power consumption while having a high resolution and large area can be made.
FIG. 1 is a circuit diagram showing a basic pixel structure of an active matrix organic light emitting diode device according to the related art. As shown in FIG. 1, a scanning line is arranged in a first direction, and a signal line and a power supply line are arranged in a second direction perpendicular to the first direction. The signal line and the power supply line are spaced apart from each other defining a pixel region therebetween. A switching thin film transistor (often referred to as a selection transistor), an addressing element, is connected to the scanning line and the signal line. A storage capacitor CST is connected between the switching thin film transistor (TFT) and the power supply line. A driving thin film transistor (often referred to as a drive transistor), a current source element, is connected to the power supply line and an organic electroluminescent (EL) diode. The storage capacitor CST is connected across the driving thin film transistor (TFT). The organic EL diode has an organic EL layer (not shown) between an anode and a cathode. The switching TFT adjusts a voltage applied to the driving TFT and the storage capacitor CST stores a charge to maintain the voltage applied to the driving TFT.
When a scan signal of the scanning line is applied to a switching gate electrode of the switching TFT, the switching TFT is turned ON, and an image signal of the signal line is applied to a driving gate electrode of the driving TFT and the storage capacitor CST through the switching element. As a result, the driving TFT is turned ON. When the driving TFT is turned ON, a current of the power supply line is applied to the organic light emitting diode through the driving TFT. As a result, light is emitted. The current density of the driving element is modulated by the image signal applied to the driving gate electrode. As a result, the organic light emitting diode can display images having multiple levels of gray scale. Moreover, since the voltage of the image signal stored in the storage capacitor CST is applied to the driving gate electrode, the current density flowing into the organic light emitting diode can be maintained at a uniform level until the next image signal is applied even when the switching element is turned OFF.
FIG. 2 is a schematic plan view of an active matrix organic light emitting diode device according to the related art. As shown in FIG. 2, the active matrix organic light emitting diode device includes, for example, inverted stagger type thin film transistors. A gate line 12 crosses a data line 36 and a power supply line 34, which are spaced apart from each other. A pixel region is defined between the gate line 12 and the spaced apart data line 36 and power supply line 34. A switching thin film transistor (TFT) TS is disposed adjacent to where the gate line 12 and the data line 36 cross each other. The switching TFT TS includes a switching gate electrode 14 extending from the gate line 12, a switching source electrode 26 extending from the data line 36, a switching drain electrode 30 spaced apart from the switching source electrode 26, and a switching semiconductor layer 22 having an island shape above the switching gate electrode 14.
A driving TFT TD is connected to the switching TFT TS and the power supply line 34. The driving TFT TD includes a driving gate electrode 16, a driving source electrode 28, a driving drain electrode 32 and a driving semiconductor layer 24. The driving gate electrode 16 is connected with the switching drain electrode 30 and formed of the same material as the gate line 12 in the same fabrication step. The driving source and drain electrodes 28 and 32 overlap side portions of the driving gate electrode 16, and are formed of the same material as the data line 36. The driving semiconductor layer 24 having an island shape is disposed above the driving gate electrode 16 between the driving source and drain electrodes 28 and 32.
As also shown in FIG. 2, a power electrode 44 extends from the power supply line 34 and is connected with the driving source electrode 28. A first electrode 54 of the organic light emitting diode is disposed in the pixel region and connected to the driving drain electrode 32. A portion of the power supply line 34 is used as a first capacitor electrode for the storage capacitor CST. Further, the storage capacitor CST also includes a second capacitor electrode 42 that extends from the switching drain electrode 30. More particularly, the area where the second capacitor electrode 43 overlaps the power supply line 34 constitutes the storage capacitor CST.
FIG. 3 is a schematic cross-sectional view taken along line I—I of FIG. 2. Hereinafter with reference to FIG. 3, the driving gate electrode will be referred to as a gate electrode, the driving source electrode as a source electrode, the driving drain electrode as a drain electrode, and the driving semiconductor layer as a semiconductor layer. As shown in FIG. 3, a driving thin film transistor (TFT) TD includes a gate electrode 16, a semiconductor layer 24, and source and drain electrodes 28 and 32 over a substrate 10. The gate electrode 16 is disposed on a substrate 10. A gate insulating layer 20 is formed on the substrate covering the gate electrode 16. An active layer 24a and an ohmic contact layer 24b are formed on the gate insulating layer 20 and over the gate electrode 16. The active layer 24a and the ohmic contact layer 24b constitute the semiconductor layer 24. Spaced apart source and drain electrodes 28 and 32 are formed over the semiconductor layer 24 and respectively contact the source and drain through ohmic contact layer 24b. A portion of the ohmic contact layer 24b between the source and drain electrodes 28 and 32 is removed to form a channel region by exposing a portion of the active layer 24a. 
As also shown in FIG. 3, an interlayer insulator 48 is formed to cover the driving TFT TD. The interlayer insulator 48 has a source contact hole 46 therein which expose a portion of the source electrode 28. A power electrode 44 that extends from the power supply line 34 is formed on the interlayer insulator 48, and contacts the source electrode 28. A passivation layer 52 is formed on the interlayer insulator 48 that covers the power electrode 44. A portion of the interlayer insulator 48 and a portion of the passivation layer 52 are etched to have a drain contact hole 50 that exposes a portion of the drain electrode 32. A first electrode 54 of the organic EL diode is formed on the passivation layer 52 to connect to the drain electrode 32. As described above in reference to FIG. 2, the first electrode 54 is disposed in a pixel region.
In the active matrix type organic light emitting diode device of the related art, as is widely known, the driving TFT TD is continuously under a direct current (DC) stress. Therefore, the electrical characteristics of the driving TFT deteriorate because charge trapping or/and other defects occur in the driving TFT. Accordingly, the life span of the driving TFT decreases. Since the gate insulating layer 20 is formed by the Plasma Enhanced Chemical Vapor Deposition (PECVD) method, the gate insulating layer 20 does not properly cover steps of the gate electrode 16. Accordingly, a plurality of voids are generated in portions II of the gate insulating layer 20 where the gate insulating layer 20 covers the steps of the gate electrode 16, as shown in FIG. 3. Thus, when the direct current (DC) is applied to the driving TFT for a relatively long time, the step portions II may further deteriorate or be further damaged.
FIG. 4 is a schematic cross-sectional view illustrating an organic light emitting diode device according to the related art. Although FIG. 4 only shows two pixels in which each has three sub-pixels, this schematic is only a conceptual illustration and there will be a lot of pixels in the organic light emitting diode device. As shown in FIG. 4, first and second spaced apart substrates 70 and 90, which have inner surfaces facing each other, have a plurality of sub-pixel regions. An array layer 80 including a driving thin film transistor (TFT) TD in each sub-pixel region is formed on an inner surface of the first substrate 70. A first electrode 72 connected to the driving TFT TD is formed on the array layer 80 in each pixel region. Red, green and blue organic electroluminescent (EL) layers 74 are alternately formed on the first electrode 72. A second electrode 76 is formed on the organic EL layers 74. The first and second electrodes 72 and 76, and the organic EL layer 74 interposed therebetween constitute an organic EL diode E. The organic EL device shown in FIG. 4 is a bottom type where light is emitted from the organic EL layer 74 through the first electrode 72 and out of the first substrate 70.
The second substrate 90 is used as an encapsulation substrate. The second substrate 90 has a concave portion 92 at its inner center. The concave portion 92 is filled with a moisture absorbent desiccant 94 that removes moisture and oxygen to protect the organic EL diode E. The inner surface of the second substrate 90 is spaced apart from the second electrode 76. The first and second substrates 70 and 90 are attached with a sealant 85 at a peripheral portion of the first and second substrates 70 and 90 for encapsulation.
In an organic light emitting diode (OLED) device according to the related art, a TFT array part and an organic electroluminescent (EL) diode are formed over a first substrate, and an additional second substrate is attached to the first substrate for encapsulation. However, when the TFT array part and the organic EL diode are formed on one substrate in this way, production yield of the organic ELD is determined by a multiplication of the TFT's yield together with the organic EL diode's yield. Since the organic EL diode's yield is relatively low, the production yield of the overall OLED device becomes limited by the organic EL diode's yield. For example, even when a TFT is well fabricated, the OLED device using a thin film of about 1000 angstroms (Å) thickness can be judged as bad due to the defects of an organic electroluminescent layer. This results in loss of materials and increased production costs.
In general, the OLED devices are classified into bottom emission types and top emission types according to an emission direction of light used for displaying images via the organic ELDs. Bottom emission type OLED devices have the advantages of high encapsulation stability and high process flexibility. However, the bottom emission type OLED devices are ineffective for high resolution devices because they have poor aperture ratios. In contrast to bottom emission type OLED devices, top emission OLED devices have a higher expected life span because they have simpler circuit layouts that still yield a high aperture ratio. However, in top emission type OLED devices, the cathode is generally formed on an organic electroluminescent layer. As a result, transmittance and optical efficiency of a top emission type OLED device are reduced because of a limited number of materials that may be selected as the cathode. If a thin film-type passivation layer is formed on the cathode to prevent a reduction of the light transmittance, the thin film-type passivation layer can still fail in preventing the infiltration of exterior air into the organic electroluminescent layer.